News Paleontology

Mammal ancestors’ shrinking inner ears may reveal when warm-bloodedness arose

Fossil analysis shows a shift suggesting a sharp rise in body temperature 233 million years ago

an illustration of a mammal ancestor, with a rodent-like head and long furry body, exhaling hot air on a cold night

A mammal ancestor breathes out hot air on a cold night, suggesting that it is warm-blooded, in this artist’s rendition.

Luzia Soares

By Carolyn Gramling

Hot or not? Peeking inside an animal’s ear — even a fossilized one — may tell you whether it was warm- or cold-blooded. Using a novel method that analyzes the size and shape of the inner ear canals, researchers suggest that mammal ancestors abruptly became warm-blooded about 233 million years ago, the team reports in Nature July 20.